Find out more about the Nature of Britain in an eight-part series on BBC One - starting 10 October 2007.

Evil weed

The alien invader is Azolla filiculoides - a non-native floating water fern, also known as fairy moss.

Green carpet: the pond in Chorlton Ees

It's used in ornamental gardens and looks harmless enough - but don't be fooled. Azolla has spread into local wildlife ponds. And it’s a killer.

Jon Follows is on Manchester City Counci's environmental strategy team: "It’s pretty bad stuff. Azolla completely chokes other wildlife because it covers the surface and removing all the oxygen from the water. It even affects larger wildlife like ducks because there is nothing for them to feed off."

For some years, a seasonal pond in the Chorlton Ees Local Nature Reserve has had a thick green carpet of this floating weed, suffocating all other forms of life.

To date, it’s proved almost impossible to get rid of. Scooping the Azolla fronds off the surface only has a short term effect as the plant grows back from spores lying on the bottom of the pond.

Weevil

But on 7 September 2007, a group of special forces was brought in to tackle this alien invader. One thousand tiny weevils - a type of beetle - were unleashed with the help of youngsters from the home education group Chasing Rainbows.

Chasing Rainbows: and releasing weevils

And it’s highly effective. Both adult and larval stages of the weevil (Stenopelmus rufinasus) feed voraciously on the plant. And it’s safe: studies have shown that the weevil will not eat any other plant.

At just 2mm long, each tiny weevil cannot make much of a difference on its own. However, en masse, the weevils can be devastating. Weevils are capable of rapid reproduction and within a matter of weeks weevil populations can reach several million.

Habitat

Jon says by adopting this form of biological control, they hope to reclaim a freshwater habitat without the use of pesticides and encourage the return of native species, including frogs and newts.

“Apparently, the weevils can munch their way through the fern in a matter of weeks. So we're watching closely to see what happens.”

Councillor Neil Swannick, Executive Member for the Environment, said: "The weevils are probably the smallest ‘employees’ the council has ever used to solve a problem. We expect the weevils to get rid of the water fern very quickly and efficiently, making the pond much better for other wildlife."